Rakov Skocjan is the Karst valley of the brook Rak. The Big Natural Bridge is the remainder of the cave roof from the distant past, when the entire stream of the water was under the land. The central part of the valley, where the sank began, is the oldest. In the original and sinkhole part is the melting of the rocky stratums continuing. The bridge is built from beautifully stratified limestone, which was taking shape in the bottom of the former sea more than 100 million years ago. The strong tectonic powers lifted this stratums and juted them out. The lower, mechanically crushed part of the fault water later excavated and shaped into the present image. On the wall can be seen expressive black, yellow and red spots. These are different sorts of algae and crust lichens, who can only survive in this uneasy place to live. The Big Natural Bridge is 10 metres high and 40 metres long arch. Through it flows the river Rak. On the drain side is 37 metres high, wide is enough, so the road can go here from the western side to the restaurant.